1. Field of the Art
The present invention relates in general to a soft contact lens, and more particularly to an improvement in the configuration of a back surface of the soft contact lens, the back surface serving as a cornea-contacting surface which contacts a cornea of a human eye.
2. Discussion of the Prior Art
The configuration of a contact lens should be determined so as to satisfy the following three major requirements A through C, for assuring a contact lens wearer of comfortable wearing conditions of the lens, i.e., good adaptability and compatibility properties of the lens. The three major requirements are as follows.
A: The back surface of the contact lens is formed in a concave shape, so that the back surface of the lens may suitably fit the shape (convex) of the cornea of the lens wearer.
B: The configuration of the peripheral portion of the lens permits good circulation of the tear fluid existing between the cornea and the back surface of the lens.
C: The lens as a whole has a smooth surface and a small wall thickness, to thereby decrease discomfort as felt by the contact lens wearer when he blinks.
It is known that the surface of the human cornea is not a truly spherical surface, but an aspherical surface whose curvature generally increases from its center toward its periphery. In view of this fact, various attempts have been made to adapt the configuration of the back surface of the contact lens so as to satisfy the above-mentioned requirement A.
More specifically described, a contact lens having an ellipsoidal back surface has been proposed since the configuration of the cornea of the human eye approximates an ellipsoidal surface. For example, U.S. Pat. No. 3,482,906 to David Volk discloses a hard contact lens whose entire back surface is formed by a single ellipsoidal surface. Although the disclosed contact lens permits better wearing conditions as compared with a contact lens whose back surface is made spherical, the contact lens does not necessarily satisfy the requirement A as indicated above, that is, the back surface of the contact lens does not closely fit the cornea, because of its single ellipsoidal back surface configuration.
Described in detail, the curvature of the cornea changes such that the radius of curvature of the cornea increases from its center toward its periphery as stated above, but the rate of change in the curvature is not constant. If the entire back surface of the contact lens is formed by adopting an eccentricity which is determined based on the rate of change in the curvature at the central portion of the cornea, the peripheral portion of the back surface of the contact lens contacts too closely to the cornea, placing the contact lens in a so-called "steep" condition in which the central portion of the contact lens is spaced apart from or floats above the cornea. On the other hand, if the back surface of the lens is formed to have an eccentricity determined based on the rate of change in the curvature at the peripheral portion of the cornea, the lens is placed in a so-called "flat" condition wherein the peripheral portion of the lens floats away from the cornea.
Further, the contact lens disclosed in the above publication is not satisfactory in achieving the configuration of the peripheral portion of the lens to satisfy the above-mentioned requirement B, that is, to permit good circulation of the tear fluid between the back surface of the lens and the cornea since the peripheral portion of the lens is apart from the cornea by a great distance, making it difficult to obtain a suitable clearance between the lens and the cornea. When the clearance between the peripheral portion of the lens and the cornea is too large, it gives rise to problems that the lens wearer feels more frequently the feeling of discomfort when the wearer blinks and that the lens tends to slip off from the eye due to the blinking.
For avoiding the inconveniences as stated above, there is a need to prepare a considerably large variety of lenses for fitting the individual corneal shapes of the wearers. In fact, the above-mentioned publication discloses that the various kinds of lenses are prepared which have various shapes and sizes. However, it is not desirable to prepare such a wide variety of lenses, in view of the productivity of the lens.
The above problems are solved to some extent by a contact lens as proposed in U.S. Pat. No. 4,883,350 to Dieter Muckenhirn wherein the central portion of the back surface of the lens (i.e., the angular area ranging between 10.degree. and 20.degree. measured from the center of curvature of the back surface of the lens) is made spherical, whereas the peripheral portion adjacent to the central spherical portion is made aspherical, such as elliptical. However, the disclosed lens fails to satisfy the requirement A indicated above since the spherical surface of the central portion of the contact lens is not compatible with the aspherical surface of the cornea which is approximately ellipsoidal. The publication explains that the central portion of the back surface of the lens is made spherical to provide an accurate dioptric power of the lens. Yet because of the central spherical portion, the lens tends to be adversely influenced by spherical aberration.
Still another example is disclosed in U.S. Pat. No. 4,765,728, wherein the back surface of the lens includes a central zone constituted by a first second-order surface of revolution other than spherical, and a marginal zone constituted by a second second-order surface of revolution other than spherical, which second second-order surface is different from the first second-order surface of revolution. In this contact lens, the transition from the first second-order surface of revolution to the second second-order surface of revolution lies on a tangent common to both curves of the two surfaces, and an axial clearance between the cornea and the back surface of the lens increases toward the edge of the lens as the marginal zone of the lens gradually separates from the cornea. However, if the configuration of the back surface according to this arrangement is applied to a soft contact lens, it would cause a problem that the lens is likely to be positioned away from the cornea over its area ranging from the peripheral portion through the central portion (serving as the optical portion) thereof, leading to deterioration of the wearing conditions of the lens and slipping of the lens from the cornea.
As is apparent from the above explanation, any one of the conventionally proposed configurations of the back surface of the contact lens does not assure the contact lens wearers of comfortable wearing conditions when the proposed configurations of the back surface are directly applied to a soft contact lens.